The present invention relates to a power tool, in particular a hand-operated chiseling power tool and a control method for the power tool.
In the case of hand-held chiseling power tools, chiseling action is supposed to be suspended when a chisel is lifted off a workpiece. In the case of striking mechanisms that operate pneumatically, a pneumatic spring can be deactivated by means of additional ventilation openings, which are only opened if the chisel is disengaged. A striker, also called an intermediate striking device or anvil, is supposed to remain away from the ventilation openings for this purpose after an empty impact. However, this is not the case to some extent due to the rebound of the striker on a forward limit stop.
A power tool according to the invention has a striker, which is guided along an axis parallel to an impact direction. A pneumatic chamber has a volume which varies with a movement of the striker along the axis. A valve device that is actuatable depending upon the movement direction of the striker connects the pneumatic chamber with an air reservoir. The valve device is actuated to open in the case of a movement of the striker in the impact direction and in the case of a movement of the striker against the impact direction is actuated to throttle or close. The throttled or closed valve device restricts an air flow flowing through it to a maximum of one tenth of the value as compared to the air flow in an opened position.
The striker is an impact body or anvil that is moveable along an axis, which is arranged between a striking device of a pneumatic striking mechanism and a tool inserted into a tool receptacle.
The striker experiences a braking effect because of the closed pneumatic chamber when it slides back into the tool receptacle. In the case of a movement in the impact direction, the valve device makes a pressure equalization possible in the pneumatic chamber, which is why no braking effect occurs.
One embodiment provides that the volume of the pneumatic chamber is preferably increasing monotonically in the case of a movement of the striker in the impact direction and the valve device is open for an air flow into the pneumatic chamber and throttled or blocked for an air flow out of the pneumatic chamber. Another embodiment provides that the volume of the pneumatic chamber is, for example, decreasing monotonically in the case of a movement of the striker in the impact direction, and the valve device is throttled or blocked for an air flow into the pneumatic chamber and open for an air flow out of the pneumatic chamber. The air reservoir may be a further pneumatic chamber, whose volume is, for example, decreasing monotonically in the case of a movement of the striker in the impact direction and the valve device connects the pneumatic chamber with the further pneumatic chamber. The actuated opened valve device may connect the pneumatic chamber with the further pneumatic chamber in such a way that an air quantity escaping from the further pneumatic chamber flows into the pneumatic chamber. One or two pneumatic chambers may be provided, which compress or expand in the case of a movement in the impact direction depending upon their relative arrangement with respect to the striker. A valve device may be provided for each of the chambers or even in the case of two chambers these are connected via a common valve device.
One embodiment provides that the pneumatic chamber is closed by a guide for guiding the striker along the axis, the striker and two seals arranged offset from one another along the axis, e.g., in the radial direction, between the striker and the guide, wherein in a projection onto a plane perpendicular to the axis, the two seals do not overlap at least in sections.
One embodiment provides that the pneumatic chamber and the additional pneumatic chambers are closed by a guide for guiding the striker along the axis, the striker and three seals arranged offset from one another along the axis between the striker and the guide, wherein the respective adjacent seals in a projection onto a plane perpendicular to the axis do not overlap at least in sections. At least one of the seals may be formed by the valve device. An opening in the guide may be arranged between two adjacent seals, and the valve device connects the opening with the air reservoir or a further air reservoir. The valve device may be arranged outside of the guide.
One embodiment provides that the valve device is a valve device actuated by its own medium, which is actuated by an air flow into or out of the pneumatic chamber. An air flow keeps the valve device open when the airflow flows in the flow-through direction. An air pressure, which acts against the flow-through direction on the valve device, closes it. The valve device may include a check valve.
One embodiment has a throttle, which connects the pneumatic chamber with an air reservoir. An effective cross-sectional area of the pneumatic chamber, defined by the differential of the volume of the pneumatic chamber in the impact direction is greater than a hundred times a cross-sectional area of the throttle. The striker is moved parallel to the axis, whereby a volume change of the pneumatic chamber is produced proportional to the displacement along the axis and the effective cross-sectional area. The effective cross-sectional area can be determined by the mathematical operation of differentiation in the movement or impact direction. In the case of a cylindrical guide and a cylindrical striker, the effective cross-sectional area corresponds to the largest cross-sectional area perpendicular to the axis. The ratio of the effective cross-sectional area of the pneumatic chamber to the cross-sectional area of the throttle determines a relative flow speed of the air in the throttle related to the speed of the striker. Starting at this relative flow speed, the air can escape quickly enough from the pneumatic chamber without a drop in pressure developing with respect to the environment. It was recognized that an absolute speed of the air in the throttle cannot be exceeded. However, the throttle appears to block a limit value of the absolute speed. The ratio of a hundred times, preferably three-hundred times, is selected so that, in the case of a striker driven by the striking mechanism, the absolute speed of the air in the throttle is reached; in the case of a striker moved manually, the absolute speed is fallen short of considerably. As a result, the throttle blocks when the striker strikes, and opens when the striker is moved manually.
In one embodiment, the power tool has a pneumatic striking mechanism, which is arranged percussively with its impacting piston in the impact direction on the striker.
In the case of a control method according to the invention for the power tool, the valve device is opened if the striker moves in the impact direction, and the valve device is closed if the striker moves against the impact direction.
The following description explains the invention on the basis of exemplary embodiments and figures.